This invention relates to capstan drive systems for tapes such as are used in magnetic tape read and write functions. The present system is especially suited for use with magnetic tape duplicating equipment which operate at high speeds, such as 240 inches per second.
A tape drive includes a magnetic head, an ingoing capstan upstream of the head, and an outgoing capstan downstream of the head. The tape moves between the ingoing capstan and a pinch roller, over the head, and between the outgoing capstan and a pinch roller. A small and controlled amount of tension is desired in the tape between the two capstans. The speed differential typically is achieved by driving the two capstans at the same rotational speed, with the outgoing capstan of a larger diameter than the ingoing capstan thereby providing a higher linear speed at the outgoing capstan. Changing of the speed differential between the two capstans requires changing of the outside diameter of one or both of the capstans. This requires a supply of capstans or capstan sleeves, and considerable effort on the part of the operator to make a change. Also, the drive motors typically are synchronous type motors with the rotatational speed being line frequency dependent with resultant undesirable changes in tape speed past the head.
It is an object of the present invention to provide a new and improved capstan drive system which utilizes dc motors that are not line frequency dependent thereby providing improved tape speed control. A further object is to provide such a drive system which uses identical mechanical parts for the ingoing and outgoing capstans, with the tape speed differential being achieved by driving the capstans at different rotational speeds. A particular object is to provide such a capstan drive system where the operator may change the speed differential and thereby change the tape tension by operating a selector switch.
It is an object of the invention to provide a capstan drive system incorporating separate servo-controls for each drive motor, preferably phase lock loop servo-circuits, with each circuit controlled by a reference frequency. A further object is to provide such a system utilizing a stable oscillator which provides the reference frequency for one servo-circuit and the reference frequency for a third phase lock loop circuit which in turn generates the other reference frequency as a function of an operator selected frequency divider circuit in the phase lock loop.
Other objects, advantages, features and results will more fully appear in the course of the following description.